User blog comment:KirLudwig/Ralts/@comment-32358261-20180225035631/@comment-32358261-20180225201103

Pretty much each Pokémon have different catch rates with a max at 255. For example, most legendary Pokémon have a catch rate of 3. Rare Pokémon is usually around 45.

Then it takes in the pokeballs capture multiplier. Now include hp, status and other extra multipliers (like capture powers and stuff).

Once all these variables are taken, it’s put into a formula.

Then if number is higher than a specific value, it will be caught.